Text sizing and all sorts of other styling are handled in the skin.css file that is part of the site skin. It is possible, too, to add styling in a container.css file that goes with a particular container. The module title is controlled by the Head class, unless you redefine it in your container.css file.
My recommendation to you is to study some of the tutorials related to skin construction. There are so many good tutorials about skinning that I'll just refer you to
this page. And don't forget to take a look at the
DotNetNuke Skinning Toolkit. Everything is in there.
Thanks to Lee, I also learned about the Web Developer's Toolbar for Firefox. It allows you to edit a site's CSS file (and about a hundred other things) so you can play around with it and watch your site change right in the browser window. Check
How to test a skin (I think) for more information about that tool.
And, continue to ask questions here.